Think about where you fit into the organization.
Most administrators will manage resources utilized by the user population, for example, create user accounts, reset passwords, and manage mail accounts. A background in Microsoft Exchange is generally required for these positions. (MCSE, MCSA or MCP certification helps.)
Some administrators are more involved with the infrastructure, for example, they will setup the Active Directory (AD), DHCP, and DNS servers. These administrators may need some familiarity with routers and switches, especially with Cisco hardware and software. (MCSE and CCNA certification helps.)
Another category I'm starting to see is an administrator with some programming experience. Scripting skills are a big plus in large enterprises where much of the administration is automated with custom tools. Knowing how to use VBScript, PowerShell, or WinNT shell command batch files can give you an edge over others.